1. Field of the Invention
In a broad aspect, this invention relates to removal of fluoride from waste streams, particularly spent aqueous hydrogen fluoride waste water from catalytic alkylation units. In a specific aspect, the invention provides a method for removing dissolved fluoride with an enhanced calcium fluoride precipitate that may be continuously recycled for controlled particle growth.
2. Related Art
Many processes require fluoride, as hydrofluoric acid or in combination with other acids. For example, hydrofluoric acid is used in surface treatment of metals as part of finishing procedures. Hydrofluoric acid is also used as a catalyst in the conversion of normal octane gasolines to highly branched isomers such as iso-octane. Industry has recognized that removal of dissolved fluoride from these waste water streams is generally a desirable and often necessary procedure.
The occurrence of fluoride in these waste streams can vary from a few milligrams per liter to tens of percent. Other dissolved contaminants typically include metals, such as iron, chromium, nickel and manganese balanced with fluoride ions (F.sup.-). These metals are often balanced with other acids, such as nitric acid (HNO.sub.3), hydrochloric acid (HCl), and sulfuric acid (H.sub.2 SO.sub.4). The acids are typically present in excess of quantities that would be required to balance the metals, creating a low pH solution and assuring that all of the metals are soluble. Substantially all of the contaminants are dissolved in the waste waters, such that both the metals and the balancing ions are in the ionic form (e.g., iron as Fe.sup.+3 or Fe.sup.+2 chloride as Cl.sup.-, fluoride as F.sup.-, etc.).
While conventional methods are available for removing fluoride from waste water, these methods have a number of shortcomings. While most metals contained in waste waters will precipitate as an insoluble hydroxide precipitate, fluoride, itself being an anion, will not react with hydroxide to form an insoluble compound. Fluoride will, however, react with the calcium in lime, a common reagent used for neutralization of waste waters. In that case, insoluble calcium fluoride (CaF.sub.2) is formed and the fluoride removed as solid precipitate. Other alkaline species, such as sodium hydroxide (NaOH), potassium hydroxides (KOH) and ammonium hydroxide (NH.sub.4 OH), cannot remove the fluoride.
Ferric sulfate [Fe.sub.2 (SO.sub.4).sub.3 ] forms trivalent iron ions (Fe.sup.+3) and divalent sulfate ions (SO.sub.4.sup.-2) in waste water. Similarly, hydrofluoric acid forms hydrogen ions (H.sup.+) and fluoride ions (F.sup.-). The ferric hydroxide [Fe(OH).sub.3 ] and calcium fluoride (CaF.sub.2) are highly insoluble and tend to form solid precipitates. Other metals present also form insoluble hydroxides, much the same as the ferric iron. The metal hydroxide and calcium fluoride precipitates may be separated from the remaining water with a settling device, such as a thickener. The settling device may produce a sludge containing the settled precipitates and a water effluent that is relatively free of suspended and dissolved metals and fluoride. Disadvantages of conventional systems include the frequent need for large and/or numerous expensive thickeners because of the need for a large settling area.
Spent hydrofluoric acid catalysts from refinery alkylation units typically are discharged as a neutral to alkaline solution, e.g., pH of 7 to 13, resulting from pretreatment of the hydrofluoric acid with NaOH or KOH. This neutralized spent catalyst solution may contain around 3 to 10 percent F.sup.- in the form of NaF and/or KF, which is very concentrated from a treatment standpoint. Caustic materials are added to neutralize the free acidity and thus reduce the hazardous nature of the catalyst solution. The concentrated catalyst solution often requires further treatment with a calcium compound to precipitate the fluoride. The treatment results in a highly voluminous and gelatinous precipitate that is difficult to settle and dewater. The usual approach has been to dewater the material by using high pressure filters and employing very long cycle times or by using large area settling ponds. It is therefore desirable to develop a process which will require minimal or no thickening and which preferably produces a concentrated, non-gelling precipitate that can be dewatered efficiently to a highly concentrated cake. Such a process would also permit more efficient recycling of the calcium fluoride product for industrial uses, such as regenerating the HF for reuse as a catalyst in an alkylation reaction.